As is known in the art, low noise amplifiers are used in a wide range of applications. One such application is in a phased array systems having an array of transducers such as, for example, radio frequency antennas of the type used in radar systems or sonic transducers of the type used in sonar and ultrasound systems. In such phased array systems the amplifiers are included in a beam forming network. More particularly, the beam forming network includes a plurality of the low noise amplifiers, each one connected to a corresponding one of the transducers, and a plurality of phase shifters each one connected to a corresponding one of the amplifiers. A controller provides phase shifting signals to the phase shifter to provide a collimated and directed beam of energy; radio frequency energy in the case of an antenna and sound or ultrasound energy in the case of sonic transducers.
Gain control signals are provided to adjust the gain of the amplifiers, for example as a function of time after transmission of a pulse of energy to adjust for changes in the strength of a signal received by the transducers in response to such transmitted pulse, i.e., Time-Gain Compensation (TGC). In some systems, the variable gain amplifier (VGA) used by the TGC is performed using interpolative methods. These methods generally require relatively large integrated circuit die area. In addition, such method requires a relatively complex controller to generate the different levels of interpolation. Also, when passive type interpolation is used, the noise figure (NF) generally begins to degrade immediately as the gain is reduced from the TGC setting. This makes it difficult to use in a low gain, low noise mode by adjusting the TGC setting. Another type of variable gain amplifier (VGA) used to perform TGC includes current steering to a differential pair of transistors. Here, a bias current for the differential pair is steered from one of the transistors to the other one of transistors thereby causing a change in amplifier gain. While such VGA does not require a relatively complex controller, and is very efficient in its use of chip area; only one differential pair is needed. However, the output compression of the amplifier decreases with reduction in gain thereby limiting the usable dynamic range of the amplifier. While the dynamic range may be increased by using an adjustable bias for the differential pair which varies with gain, such adjustable bias increases complexity, and power requirements, of the amplifier.